They day Johnson signed the Civil Rights act.
There is a lot more to it than that, spidey. But since it fits your ideology, believe it.
Blacks began to abandon the Republican party during the time of FDR because the depression was generally blamed on the Republicans and the Democrats promised a better economy. Many whites did the same. So the racial element was generally a wash.
As the Democrats drifted further and further left during the Vietnam war, white southern Democrats, who were more pro-war felt increasingly isolated by and within their party. They were also frightened by the runway spending on social programs such as the Great Society and the War on Poverty.
The GOP welcomed the defectors primarily based on the promise of fiscal conservativism. Was race a factor? Probably for some, but as much as spidey wants to castigate all Republicans as racists, it was not the primary factor in the cross over.
As in any political movement, there were many variables, but the politically immature canrely on the race card. It's easier and requires no for or after thought. And makes for sparkling one liners.